Strontium hexaferrite platelets: a comprehensive soft X-ray absorption and Mössbauer spectroscopy study

IBERMÖSS-2019, Bilbao, 30-31 may 2019. --https://www.ehu.eus/es/web/ibermossmeetingStrontium ferrite (SFO, SrFe12O19) is a ferrite employed for permanent magnets due to its high magnetocrystalline anisotropy. Since its discovery in the mid-20th century, this hexagonal ferrite has become an increasingly important material both commercially and technologically, finding a variety of uses and applications. Its structure can be considered a sequence of alternating spinel (S) and rocksalt (R) blocks. All the iron cations are in the Fe3+ oxidation state and it has a ferrimagnetic configuration with five different cationic environments for the iron (three octahedral sites, a tetraedrical site and a bipiramidal site)[1,2]. We have studied the properties of SrFe 12O19 in the shape of platelets, up to several micrometers in width, and tens of nanometers thick, synthesized by a hydrothermal method. We have characterized the structural and magnetic properties of these platelets by Mössbauer spectroscopy, x-ray transmission microscopy (TMX), transmission electron microscopy (TEM), x-ray diffraction (XRD), vibrating-sample magnetometry (VSM), x-ray absorption spectroscopy (XAS), x-ray circular magnetic dichroism (XMCD) and photoemission electron microscopy (PEEM). To the best of our knowledge this is the first time that the x-ray absorption spectra at the Fe L 2,3 edges of this material in its pure form have been reported. The Mössbauer results recorded from these platelets both in the electron detection and transmission modes have helped to understand the iron magnetic moments determined by XMCD (Fig.1). The experimental results have been complemented with multiplet calculations aimed at reproducing the observed XAS and XMCD spectra at the Fe L 2,3 absorption edge, and by density functional theory (DFT) calculations to reproduce the oxygen K- absorption edge. Finally the domain pattern measured in remanence is in good agreement with micromagnetic simulations [3]

Ab-initio study of magnetic properties of complex oxides based on ferrites

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, leída el 12-12-2022Permanent magnets are nowadays regarded as essential, meaning they cannot be replaced by others materiales, for many different technological applications. The most powerful magnets contain rare-earths, elements that are ay supply risk for the European Union and other regions, as China controls more then 80% of their global production...Los imanes permanentes se consideran hoy día esenciales, es decir, no pueden ser sustituidos por otros materiales, para muchas aplicaciones tecnológicas diferentes. Los imanes más potentes contienen tierras raras, elementos que suponen un riesgo de suministro para la Unión Europea (UE) y otras regiones, ya que China controla más del 80% de su producción mundial...Fac. de Ciencias FísicasTRUEunpu

Estudio de primeros principios de propiedades magnéticas en óxidos complejos basados en ferritas

[EN] This PhD thesis investigates paths to improve the performance of nanostructured bicomponent permanent magnets (PMs) based on ferrites, using ab initio simulations of their magnetic properties. We focus on ferrites as an alternative to avoid rare-earths, whose extended use represents a serious economic, health and social problem. On one hand, the research work addresses specific practical problems of solutions devised to improve the performance of the nanostructured ferrite PMs. This includes evaluation of the magnetization of Ni ferrite upon inclusion of substitutional Zn atoms, or the role of the interfaces between cubic ferrites in the magnetic response of bicomponent systems that combine hard and soft magnets. On the other, the work tackles fundamental theoretical problems, identifying the relevance of different approximations inherent to the ab initio determination of the magnetic exchange constants of both hexagonal and cubic ferrites. This determination is at the basis of multiscale magnetic models widely used to simulate the magnetic response of nanostructured PMs. The main contributions of this work are: (i) to provide a fundamental ab initio description of hexagonal Strontium ferrite and cubic Ni, Co and Zn ferrites, both in their inverse and direct forms, identifying the large influence of the Hubbard U term on their magnetic properties; (ii) to quantify the influence of the approximations inherent to the mapping of ab initio energies to Heisenberg Hamiltonians in the values of the magnetic exchange constants; (iii) to identify the supremacy of Monte Carlo methods over mean field approaches to estimate magnetic ordering temperatures; (iv) to prove the origin of the decreased magnetization of Ni ferrite under Zn substiution, caused by the tendency of Zn to occupy octahedral sites; (v) to determine the features of the interfaces between Co ferrite and both Ni and Mn ferrites, evidencing strain as the main source of modifications, mainly regarding the magnetic anisotropy.[ES] Esta tesis investiga cómo mejorar el rendimiento de imanes permanentes (IPs) nanoestructurados basados en sistemas bicomponente de ferritas, utilizando técnicas de primeros principios. Analizamos ferritas como alternativa a los imanes basados en tierras raras, elementos que representan una seria amenaza desde el punto de vista de la sostenibilidad, la economía y la salud. Por un lado, investigamos problemas prácticos asociados a algunas de las soluciones propuestas para mejorar el rendimiento de los IPs nanoestructurados de ferritas. Esto incluye la evaluación de la imanación de la ferrita de Ni cuando se introduce Zn sustitucional, o el análisis del papel de las intercaras de ferritas cúbicas en la respuesta magnética de sistemas bicomponente que combinan imanes blando y duro. Por otra parte, nuestra investigación aborda problemas teóricos fundamentales, identificando la relevancia de las diferentes aproximaciones inherentes a la determinación de las constantes de canje de ferritas cúbicas y hexagonales. Esta determinación constituye la base de los ampliamente extendidos modelos magnéticos multiescala que simulan la respuesta magnética de IPs nanoestructurados. Las contribuciones más relevantes de esta tesis son: (i) proporcionar la descripción fundamental de primeros principios de la ferrita hexagonal de Sr y las ferritas cúbicas de Ni, Co y Zn, en sus formas directa e inversa, identificando la enorme influencia del potencial local de Hubbard (U) en sus propiedades magnéticas; (ii) cuantificar la influencia de las aproximaciones inherentes al mapeo de energías ab initio a Hamiltonianos de Heisenberg para extraer las constantes de canje magnético; (iii) establecer la supremacía de los métodos de Monte Carlo sobre las aproximaciones de campo medio en la estimación de las temperaturas de orden magnético; (iv) demostrar el origen de la reducción de imanación de la ferrita de Ni cuando se introduce Zn sustitucional, debida a la tendencia de los átomos de Zn a ocupar posiciones de coordinación octahédrica; (v) determinar las características de las intercaras de ferritas de Co con ferritas de Mn y Ni, demostrando que la mayor fuente de modificaciones proviene de la aparición de tensiones estructurales, que afectan principalmente a la anisotropía magnética.Unión Europea, Horizon 2020, Amphibian-NMBP 720853; Ministerio de Economía y Competitividad, RTI2018-097895-C41Peer reviewe

Interface effects in combined soft/hard ferrite permanent magnets

[EN] We address the detailed description of the magnetic properties of the (001) and (111) interfaces between the hard CoFe2O4 (CFO) and soft NiFe2O4 spinel ferrites, used in bicomponent systems for permanent magnet applications. The similarity between the electronic properties, magnetic order and lattice structure of both oxides allows us to isolate the effect of ideal interface formation on the magnetism of the combined system based on density functional theory. While the magnetic moments and long-range magnetic order preserve bulk-like values, the magnetic anisotropy is reduced by the presence of the interface, and depends on the specific termination. Partial inversion of CFO is also relevant, as it enhances the net magnetization and alters both the magnitude and spatial dependence of the magnetic anisotropy.This work has been financed by the European Commission under Grant H2020 No. 720853 (Amphibian) and by the Spanish Ministry of Economy and Competitiveness under Grant RTI2018-097895-C41. Parts of the simulations have been performed in the supercomputer Finis Terrae of the CESGA

Strontium hexaferrite platelets: a comprehensive soft X-ray absorption and Mössbauer spectroscopy study

MECAME / GFSM 2019, Montpellier, 19 to 23 may 2019 .-- In honour of Dr Jean-Claude Jumas (Institut Charles Gerhardt, CNRS, University of Montpellier, France). -- https://mecame-gfsm2019.irb.hr/Strontium ferrite (SFO, SrFe12O19) is a ferrite employed for permanent magnets due to its high magnetocrystalline anisotropy. Since its discovery in the mid-20th century, this hexagonal ferrite has become an increasingly important material both commercially and technologically, finding a variety of uses and applications. Its structure can be considered a sequence of alternating spinel (S) and rocksalt (R) blocks. All the iron cations are in the Fe3+ oxidation state and it has a ferrimagnetic configuration with five different cationic environments for the iron (three octahedral sites, a tetraedrical site and a bipiramidal site)[1,2]. We have studied the properties of SrFe 12O19 in the shape of platelets, up to several micrometers in width, and tens of nanometers thick, synthesized by a hydrothermal method. We have characterized the structural and magnetic properties of these platelets by Mössbauer spectroscopy, x-ray transmission microscopy (TMX), transmission electron microscopy (TEM), x-ray diffraction (XRD), vibrating-sample magnetometry (VSM), x-ray absorption spectroscopy (XAS), x-ray circular magnetic dichroism (XMCD) and photoemission electron microscopy (PEEM). To the best of our knowledge this is the first time that the x-ray absorption spectra at the Fe L 2,3 edges of this material in its pure form have been reported. The Mössbauer results recorded from these platelets both in the electron detection and transmission modes have helped to understand the iron magnetic moments determined by XMCD (Fig.1). The experimental results have been complemented with multiplet calculations aimed at reproducing the observed XAS and XMCD spectra at the Fe L 2,3 absorption edge, and by density functional theory (DFT) calculations to reproduce the oxygen K- absorption edge. Finally the domain pattern measured in remanence is in good agreement with micromagnetic simulations [3]

Strontium hexaferrite platelets: a comprehensive soft X-ray absorption and Mössbauer spectroscopy study

Platelets of strontium hexaferrite (SrFeO, SFO), up to several micrometers in width, and tens of nanometers thick have been synthesized by a hydrothermal method. They have been studied by a combination of structural and magnetic techniques, with emphasis on Mössbauer spectroscopy and X-ray absorption based-measurements including spectroscopy and microscopy on the iron-L edges and the oxygen-K edge, allowing us to establish the differences and similarities between our synthesized nanostructures and commercial powders. The Mössbauer spectra reveal a greater contribution of iron tetrahedral sites in platelets in comparison to pure bulk material. For reference, high-resolution absorption and dichroic spectra have also been measured both from the platelets and from pure bulk material. The O-K edge has been reproduced by density functional theory calculations. Out-of-plane domains were observed with 180° domain walls less than 20 nm width, in good agreement with micromagnetic simulations.This work is supported by the Spanish Ministry of Economy and Competitiveness through Projects MAT2015-64110-C2-1-P, MAT2015-64110-C2-2-P, MAT2015-66888-C3-1-R and by the European Commission through Project H2020 No. 720853 (Amphibian). These experiments were performed at the CIRCE, MISTRAL and BOREAS beamlines of the ALBA synchrotron Light Facility. G.D.S. acknowledges the European Youth Employement Initiative and the Autonomous Community of Madrid for a one-year fellowship. Slovenian Research Agency is acknowledged for funding the research program Ceramics and complementary materials for advanced engineering and biomedical applications (P2-0087), CEMM, JSI for the use of TEM

AMPHIBIAN CSIC experimental data SrFe12O19 platelets SciRep2018. AMPHIBIAN CSIC simulated data SrFe12O19 platelets SciRep2018. [Dataset]

[EN] Platelets of strontium hexaferrite (SrFeO, SFO), up to several micrometers in width, and tens of nanometers thick have been synthesized by a hydrothermal method. They have been studied by a combination of structural and magnetic techniques, with emphasis on Mössbauer spectroscopy and X-ray absorption based-measurements including spectroscopy and microscopy on the iron-L edges and the oxygen-K edge, allowing us to establish the differences and similarities between our synthesized nanostructures and commercial powders. The Mössbauer spectra reveal a greater contribution of iron tetrahedral sites in platelets in comparison to pure bulk material. For reference, high-resolution absorption and dichroic spectra have also been measured both from the platelets and from pure bulk material. The O-K edge has been reproduced by density functional theory calculations. Out-of-plane domains were observed with 180° domain walls less than 20 nm width, in good agreement with micromagnetic simulations.UE, programa H2020, Proyecto AMPHIBIAN n º 720853Peer reviewe